In touch screen modules, the FPC (Flexible Printed Circuit) serves as a critical component connecting the sensor to the main control chip. To enhance its reliability under harsh operating conditions, adding high-temperature adhesive is a common and effective design optimization solution.
Why Add High-Temperature Adhesive to FPC Flexible Circuits?1.Improve mechanical reliability: Prevent displacement, detachment of the FPC, or poor contact of the gold finger connector during vibration, bending, or assembly.
2.Moisture and corrosion resistance: Block the intrusion of water vapor, sweat, cleaning agents and other contaminants to protect circuits and solder joints, avoiding electrochemical migration and corrosion.
3.High temperature resistance: In high-temperature scenarios such as automotive and industrial control equipment (e.g., direct sunlight exposure, areas close to engine compartments), the high-temperature adhesive retains stable adhesion and prevents degumming or glue overflow common with ordinary tapes.
4.Dispensing adhesive on the bending zones of FPC (such as rotating shaft positions) can disperse stress and prevent fatigue fracture of copper foils.
5.Cover and protect exposed solder joints and components to prevent damage caused by external impact or friction.
6.Auxiliary electromagnetic shielding: Some conductive high-temperature adhesives filled with metal fillers can assist grounding and deliver partial local electromagnetic shielding performance. This complements the approach of adding shielding traces mentioned in the appendix to suppress noise interference.
Mainstream Compatible Adhesive Materials for FPC
| Types of Adhesive Materials |
Temperature Resistance Range |
Core Advantages |
Application Scenarios |
| Modified High-Temperature Epoxy Adhesive |
150°C for continuous use, 200°C for short-term exposure |
High bonding strength (≥2 N/cm), excellent insulation performance, anti-aging property and superior dimensional stability |
Pad areas, stiffener fixing areas, high-stress static areas |
| Silicone-Based High-Temperature Adhesive |
180°C continuous operating temperature, 260°C short-term peak temperature |
Excellent flexibility, strong adaptability to temperature changes, no brittle cracking at -60°C, and vibration resistance |
FPC bending areas, automotive & industrial high-temperature thermal cycling environments
|
| Polyimide (PI) Substrate High-Temperature Adhesive (Equivalent to 3M 9077 Grade) |
140°C continuous service temperature, 220°C short-term peak temperature |
Compatible with SMT reflow process, low shrinkage and slight yellowing under high temperature, with excellent lamination consistency
|
Pre-lamination before SMT, general mass production scenarios |
- Conventional acrylic adhesives with temperature resistance of 80–120°C are prohibited, as they cannot withstand SMT processes and high-temperature aging conditions.
- All adhesives shall meet halogen-free, flame-retardant and neutral curing requirements, with no acidic volatiles to prevent corrosion of FPC copper foils and touch circuits.
- The thermal expansion coefficient of adhesives shall match that of FPC PI substrates and copper foils to minimize thermal deformation deviation and prevent delamination and edge warpage.
Standard Process Flow
Substrate degreasing and cleaning → Die-cutting of high-temperature adhesive → Precise alignment lamination → High-temperature & high-pressure thermal compression curing → Normal-temperature standing aging → Full visual inspection → Reliability sampling test
Core Process Parameters1.Cleaning treatment: Ultrasonic cleaning + dust-free wiping to remove dust, oil stains and residual flux on FPC surface, ensure cleanness of bonding surface and avoid blistering caused by impurities during lamination.
2.Thermal compression parameters: lamination temperature 120–140°C, pressure 0.3–0.5 MPa, dwell time 30–60 s. Secondary pressing is available for thick stiffener areas to ensure full wetting and adhesion of adhesive layer.
3.Curing aging: Stand at room temperature for more than 2 hours after thermal compression to fully cure the adhesive layer and release internal stress, preventing subsequent deformation and edge warping.
4.SMT compatibility: Pre-laminated high-temperature adhesive can directly pass through reflow soldering (peak temperature ≤220°C) without blistering, peeling or excessive yellowing.
Reliability Test Verification Standards
1.High-temperature storage test: Constant storage at 85°C for 96 hours. No blistering, delamination or edge warpage of adhesive layer, no obvious attenuation of bonding strength, and normal touch function.
2.Low-temperature storage test: Stored at -40°C for 96 hours. No brittle cracking or peeling of adhesive layer, and FPC bending performance remains intact.
3.Temperature cycling test: Cycle between -40℃ and 85℃ with a single cycle of 60 minutes for 50 cycles in total; no delamination, cracking or functional abnormalities occur.
4.Peel strength test: Peel strength ≥1.5 N/cm under both room temperature and 100 ℃ high temperature environments, meeting the stress requirements of FPC assembly.
5.Solder resistance test: Short-time reflow at 220°C, no melting, shrinkage or adhesive overflow of the glue layer, with qualified insulation performance.
6.Damp heat aging test: 85°C, 85%RH for 48 hours; no water absorption blistering or debonding, electrical insulation performance remains normal.
Common Defect Risks
1.Adhesive overflow after high-temperature lamination contaminates pads and circuits.
2.Air bubbles trapped during lamination expand under high temperature, resulting in delamination.
3.Excessively rigid adhesive layer in bending zones causes cracking of FPC during flexing.
4.Mismatched thermal expansion of different materials leads to edge warpage after temperature variation.
Targeted Optimization Measures
1.Precision die-cutting: Precisely die-cut high-temperature adhesive in advance with a 0.2 mm avoidance gap reserved to eliminate adhesive overflow contaminating electrical areas.
2.Defoaming treatment: Adopt staged pressure application during thermal compression for slow air evacuation, and conduct re-inspection to remove bubbles after lamination to ensure zero residual air bubbles.
3.Material selection by zone: High-adhesion epoxy adhesive for static reinforcement areas, high-flexibility silicone adhesive for bending zones, balancing bonding strength and bending resistance.
4.Edge optimization: All bonding edges adopt circular chamfering and edge sealing treatment to relieve stress concentration caused by temperature changes and prevent edge warpage and delamination.
ConclusionThis solution effectively resolves common quality defects of traditional touch screen FPCs, including delamination, blistering, cracking and touch failure under high-temperature processes, alternating high and low temperature cycling, and vibration conditions, by adopting a customized high-temperature adhesive structure, optimized material selection and refined manufacturing processes. The zoned layout design of high-temperature adhesives ensures structural strength and insulation protection for the stress-bearing and high-temperature-prone areas of FPCs, while maintaining the overall flexibility of FPCs without compromising assembly accuracy and user touch feeling.
This solution is compatible with mass production requirements across multiple scenarios such as consumer electronics, automotive touch panels and industrial touch screens. Featuring mature processes and low renovation costs, it can be rapidly deployed to replace conventional general adhesive lamination solutions. It effectively improves product reliability and service life while reducing after-sales defect rates.
FAQQ1: Can this high-temperature adhesive lamination solution directly pass SMT reflow soldering?A: Yes. This solution supports pre-lamination process. The adhesive can withstand a peak temperature of 220℃. No blistering, overflow, yellowing or peeling occurs during normal reflow soldering, which fits SMT mass production procedures.
Q2: Will FPC edge warpage or delamination happen after temperature cycling?A: No. Multiple optimizations including zoned adhesive selection, edge chamfering & sealing and matched thermal expansion coefficient are applied. No delamination or warpage is observed after 50 cycles between -40℃ and 85℃, suitable for wide-temperature automotive and industrial applications.
Q3: Is the adhesive prone to cracking on bending areas?A: Hardly. High-flexibility silicone adhesive is specially used for bending zones to retain FPC flexibility. No brittle cracking of adhesive layer appears in low-temperature storage and repeated bending tests, ideal for flexible touch structures.
Q4: Does switching to this solution require high production line renovation costs?A: The renovation cost is low. Existing thermal lamination equipment does not need large-scale replacement. Only die-cutting fixtures and staged hot-pressing parameters need adjustment. The mature process enables fast mass production launch.
Q5: Will insulation and bonding performance degrade after damp heat aging?A: No obvious degradation. No water absorption blistering or debonding after 48h damp heat aging at 85℃, 85% RH. Peel strength and insulation performance remain qualified, cutting after-sales defects under outdoor and humid environments.
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